Unit 14 Bridge Introduction
桥梁概论
内容概要
桥梁是道路上跨越山谷、河流、路线等障碍物的工程结构物，桥梁工程是土木工程一个分支。本文从桥梁的定义出发，阐明了桥梁的功能，在介绍桥梁组成部分的同时，对桥梁的每一组成部分的构造特点及其作用进行的准确地阐述。并对常见的拱桥、板桥、T 梁桥、弓弦式梁桥、悬索桥、斜拉桥、钢桥等桥梁类型，逐一介绍了该类桥梁的构造特点、适用范围，并举例说明。
A. Text Bridges
A bridge is a structure providing passage over an obstacle such as a valley, road, railway, canal, river, without closing the way beneath. The required passage may be for road, railway, canal, pipeline, cycle track or pedestrians.
The branch of civil engineering which deals with the design, planning construction and maintenance of bridge is known as bridge engineering.
1. Components of a bridge
Figure 14-1 (a) shows the elevation while Fig. 14-1 (b) presents the plan of a bridge. Broadly, a bridge can be divided into two major parts: superstructure and substructure. The superstructure of a bridge is analogous to a single storey building roof and substructure to that of the walls, columns and foundations supporting it.
Superstructure consists of structural members carrying a communication route. Thus handrails, guardstones and flooring supported by any structural system, such as beams, girders, arches and cables, above the level of bearings form the superstructure.
Substructure is a supporting system for the superstructure. It consists of piers, abutments, wingwalls and foundations for the piers and abutments.
The other main parts of a bridge structure are the approaches, bearings and river training works, such as the aprons, and the rivetment for slopes at abutments, etc. Some of the important components of a bridge are explained in this section.
Piers: These are provided in between the two extreme supports of the bridge (abutments) and in the bed of the river to reduce the span and share the total load coming over the bridge. Piers are
provided with foundation which is taken below the bed of the river where hard soil is available.
Fig. 14-1 Components of a Bridge
a) Elevation of a bridge b) Plan view of a bridge
Abutments: The end supports of a bridge superstructure are called abutments. It may be of brick masonry, stone masonry, R.C.C. or precast concrete block. It serves both as a pier and as a retaining wall. The height of abutment is equal to that of the piers. The functions of an abutment are the following:
(1) To transmit the load from the bridge superstructure to the foundations.
(2) To give final formation level to the bridge superstructure.
(3) To retain earth work of embankment of the approaches.
Wing Walls: The walls constructed at both end of the abutments to retain the earthfilling of bridge approaches are called wing walls. Normally, the wing walls have steadily decreasing cross section. The design of wing walls is independent. Generally, water face of these walls is kept vertical.
Foundations: The lowest artificially built parts of piers, abutments, etc. which are in direct contact with the subsoil supporting the structure are called foundations.
The factors which affect the selection of foundation include the type of soil, the nature of soil, the type of the bridge, the velocity of water and the superimposed load on the bridge.
Well foundation is the most commonly adopted bridge foundation in India. The foundation may consist of a single large diameter well or a group of smaller wells of circular or other shapes.
Approaches: These are the lengths of communication route at both ends of the bridge. Approaches may be in embankment or in cutting depending upon the design of the bridge. It is recommended (as per Indian Road Congress) that the approaches must be straight for a minimum length of 16 m on either side of the bridge. Its function is to carry the communication route up to the floor level of the bridge.
Hand Rails and Guard Stones: Hand rails are provided on both sides of a bridge to prevent any vehicle from falling into the stream. Footpaths are also provided for pedestrians to walk along without interfering with the heavy vehicular traffic.
In order to prevent a vehicle from striking the parapet wall or the hand rails, guard stones painted white are provided along the edge of the footpaths at the ends of the road surface. Guard stones are also provided along both sides of the approach roads in filling to prevent the vehicles from toppling over the sides of the embankments.
Bearings for the Girders The longitudinal girders have to rest over the piers which bears the thrust of the load coming over them. In order that the girder ends should rest on proper seats, the same are provided with bearing blocks made of cement concrete, so that the load may be uniformly distributed over the structure on which they rest. Due to the expansion and contraction of the longitudinal girders during severe heat and cold, rollers are provided on the abutment ends to allow the movements without causing the girder to buckle.
2. Types of bridges
2.1 Arch bridge
Arch bridges are often used because of their pleasing appearance. These are more graceful and suited for deep gorges with rocky abutments. Arch bridges can be economically adopted up to a span of 250 m. In this type of bridge, the roadway is constructed on an arch which rests on piers and abutments. An example of an arch bridge is the rainbow bridge across Niagara river over a span of 290m.
The advantages of an arch bridge are: There will be no bending anywhere in the arch, vibrations due to impact forces are minimum, and pleasing appearance.
2.2 Slab bridge
This is the simplest type of R.C. bridge and easiest to construct. Slab bridges are generally found to be economical for span up to 9 m. The thickness of slab is quite considerable but uniform, thereby requiring simple shuttering. Though the amount of concrete and steel required are more, the construction is much simpler and placement of material is easy.
2.3 T-beam and slab bridge
This consists of T-beams supported over piers and abutments. The deck slab is supported over the T-beams. This type of bridge is suitable for span between 9-20 m. T-beam bridge is cheaper and requires less quantity of materials. For example, the longest R.C. T-beam bridge in India is the Advai Bridge in Goa with a pier spacing of 35 m.
2.4 Bow string girder bridge
Bow string girder bridges are economical when sufficient head room is needed under a bridge. The main components here are an arch resembling the bow and a tie beam resembling the string of the bow. As the major portion of the load will be borne by the beam, the thrust on the abutments from the arch will be limited. Hence, the abutments need not be too heavy. The roadway is actually suspended from the arch rib by means of vertical suspenders as presented in Fig.14-2. These bridges can be adopted for spans of 30-45 m.
2.5 Suspension bridge
Superstructure of a suspension bridge
consists of two sets of cables over the towers,
carrying the bridge floor by means of
suspenders as shown in Fig.14-3. This bridge
is best suited for light traffic for large spans
exceeding 600 m. These bridges are flexible
and hence the vertical oscillations will be more than the other bridges. The entire load will be
borne by the cables which are anchored to the
ground.
2.6. The cable-stayed bridge
Cable-stayed bridges are constructed
along a structural system which comprises an
orthotropic deck and continuous girders which
are supported by stays, i.e. inclined cables
passing over or attached to towers located at
the main piers. Modern cable-stayed bridges
present a three-dimensional system consisting
of stiffening girders, transverse and
longitudinal bracings, orthotropic-type deck and supporting parts such as towers in compression and inclined cables in tension. The important characteristics of such a three-dimensional structure is the full participation of the transverse construction in the work of the main longitudinal structure. This means a considerable increase in the moment of inertia of the construction which permits a reduction in the depth of the girders and economy in steel.
2.7 Steel bridges
Steel bridges are commonly used for supporting highways, water, oil or gas pipes, a railway
Fig. 14-2 R.C.C. bow string girder bridge Fig. 14-3 Stiffened suspension bridge
Fig. 14-2 Different types of steel arches track, etc. They can be classified as follows:
2.7.1 Steel Truss Bridges
Steel truss bridges are provided for long railway bridges, as they are less affected by wind pressure. It is easy to erect steel truss bridges since its component members are relatively light in weight. The primary forces in its members are axial forces. Steel truss bridges which are commonly used are the following.
2.7.2 Steel Rigid Frame Bridge
These type of bridges, carry the roadway at the top of the portal frames. No bearing and fixtures are required in such bridges. These bridges have more clearance below them and heavy abutments are not required.
2.7.3 Plate Girder Bridges
A plate girder bridge is used to carry heavier loads over longer spans. Hence, they are mainly used for railway bridges. These are used for spans up to 20 m. In order to increase the lateral stability, box girder which consists of four plates connected by angles are used.
2.7.4 Steel Arch Bridges
Steel arch bridges are constructed
where it is not possible to construct
intermediate pier. It can be used for a
very long span, i.e. up to 150 m. Steel
arches may either be of the spandrel
braced or trussed arch type as shown in
Fig.14-4.
2.7.5 Steel Bow String Girder
Bridges
In steel bow string girder bridges,
in order to bear horizontal thrust, a steel
tie is provided which joins the two ends
of an arch. In these bridges, suspenders are provided from the arch-ribs to carry the roadway.
New Words and Expressions
Passage n.通道
Pipeline n.管线
cycle track 自行车道
pedestrians n.行人
elevation n.高程
superstructure n.上部结构
substructure n.下部结构
foundation n.基础
handrail n.护栏
bearing n.支座
pier n.桥墩
abutment n.桥台
wingwall n.翼墙
approach n.引道
apron n.裙板
masonry n.砌体
retaining wall 挡墙
transmit vt.传递
subsoil n.地基
superimposed load 附加荷载
well n.井
roller n.转轴
Arch bridge 拱桥
appearance n.外观
Gorge 峡谷
Slab bridge 板桥
R.C. 钢筋混凝土
up to 直到
Shuttering n.模板
T-beam T梁
Bow string girder 弓弦梁
tie beam 系梁
Suspension bridge 悬索桥
Oscillation 振动
cable-stayed bridge 斜拉桥
three-dimensional 三维的
orthotropic a.正交的
tower n.塔
compression n.压力
tension n.拉力
moment of inertia 惯性矩
reduction n.减少
Truss n.桁架
Erect vt.架设
portal frame 门架
Rigid Frame Bridge 刚架桥
clearance n.净空
lateral a.侧面的
Notes
1.These are provided in between the two extreme supports of the bridge (abutments) and in the bed of the river to reduce the span and share the total load coming over the bridge.
设置在桥梁的两端支撑（桥台）之间及河床中，减少跨度并承受驶过桥梁的全部荷载。2. The main components here are an arch resembling the bow and a tie beam resembling the string of the bow
主要由一道类似于弓形的拱圈和一道类似于弓弦的系梁构成。
3. Cable-stayed bridges are constructed along a structural system which comprises an orthotropic deck and continuous girders which are supported by stays, i.e. inclined cables passing over or attached to towers located at the main piers.
斜拉桥按照这样一种结构体系建造，正交板和连续梁被斜拉支撑，也就是斜索通过或固定于主墩上的桥塔。
Exercises
I. True or False
1. The end supports of a bridge superstructure are called piers. ( )
2. The functions of abutment is wholly equal to that of the piers. ( )
3.Well foundation is the most commonly adopted bridge foundation in India. ( )
4. Approach’s function is to carry the communication route up to the floor level of the bridge. ( )
5. The characteristics of an arch bridge are: There will be no bending anywhere in the arch, vibrations due to impact forces are maximum. ( )
II. Reading and Comprehensions
1. Broadly, a bridge can be divided into two major parts: .
(A) pier and abutment (B) superstructure and substructure
(C) approach and foundation (D) beam and bearing
2. are provided in between the two extreme supports of the bridge (abutments) and in the bed of the river to reduce the span and share the total load coming over the bridge.
(A) piers (B) abutments
(C) foundations (D) approaches
3.The function of an abutment is not mentioned in this section is .
(A) To transmit the load from the bridge superstructure to the foundations.
(B) To give final formation level to the bridge superstructure.
(C) protecting foundation from water washing
(D) To retain earth work of embankment of the approaches.
4. is the most commonly adopted bridge foundation in India.
(A) pile foundation (B) Well foundation
(C) expanding foundation (D) natural foundation
5. The factors which affect the selection of foundation are pointed out except .
(A) type of soil
(B) name of soil
(C) type of the bridge
(D)the velocity of water and the superimposed load on the bridge.
6. the rainbow bridge across Niagara river is a .
(A) Slab bridge (B) T-beam bridge
(C) suspended bridge (D) arch bridge
7. which statement on cable-stayed bridge is not mentioned in this section.
(A) present a three-dimensional system
(B)increase in the moment of inertia of the construction
(C)permits a reduction in the depth of the girders
(D)bear considerable pressure
8. The primary forces in Steel Truss Bridges’ members are .
(A) shear force (B) tensile force
(C) axial forces (D) bending moment
9.the right statement on Steel Rigid Frame Bridge is .
(A) No bearing and fixtures are required
(B) have less clearance below them
(C) heavy abutments are required
(D) bearing and fixtures are required
10. In steel bow string girder bridges, a steel tie is provided which joins the two ends of an arch to bear .
(A) horizontal thrust (B) vertical pressure
(C) transverse deformation (D) longitudinal deformation
III. Vocabulary Practice
1. Thus handrails, guardstones and flooring supported by any structural system, such as beams, girders, arches and cables, above the level of bearings form the superstructure.
(A) capability (B) horizontal (C) elevation (D) smooth
2. These are provided in between the two extreme supports of the bridge (abutments) and in the bed of the river to reduce the span and share the total load coming over the bridge.
(A) particular (B) special (C) end (D) supreme
3. The factors which affect the selection of foundation include the type of soil, the nature of soil, the type of the bridge, the velocity of water and the superimposed load on the bridge.
(A) great (B) important (C) main (D) additional
4. Approaches may be in embankment or in cutting depending upon the design of the bridge.
(A) excavation (B) trench (C) removal (D) backfilling
5. Due to the expansion and contraction of the longitudinal girders during severe heat and cold, rollers are provided on the abutment ends to allow the movements without causing the girder to buckle.
(A) button (B) stretch (C) bend (D) shear
6. Slab bridges are generally found to be economical for span up to 9 m.
(A) as far as (B) until (C) responsible for (D) qualify with
7. The thickness of slab is quite considerable but uniform, thereby requiring simple shuttering.
(A) supporting (B) tensioning (C) compression (D) form engineering
8. Bow string girder bridges are economical when sufficient head room is needed under a bridge.
(A) house (B) first (C) clearance (D) interval
9. The main components here are an arch resembling the bow and a tie beam resembling the string of the bow.
(A) analogy (B) installation (C) artificial (D) graceful
10. These bridges are flexible and hence the vertical oscillations will be more than the other bridges.
(A) displacements (B) movements (C) vibrations (D) expanding
Ⅳ. Translating into English
1. 桥梁是一种工程结构物，它提供了跨越山谷、道路、铁路、沟渠、河流等障碍物的通道。
2. 直接与支撑结构的地基接触的桥墩、桥台等人工建筑的最低部分称为基础。
3. 拱桥由于外观优美而常被采用。在深谷的岩石台座上修建拱桥更美观并且更适合。
4. 悬索桥的上部结构为通过桥塔的两道主缆构成，通过吊杆悬吊着桥面结构。
5. 现代斜拉桥是由加劲梁、横向和纵向支撑组成的三维体系。
B. Reading material Bridge Design Concept
1. Necessity of bridges
Bridges are vital for the development of a country since these enable transporting materials from one area to the other which may be separated by streams and rivers, thereby maintaining uniform flow of essential goods for development. In times of war, materials are swiftly transported for the defence of the country by rail road bridges. Bridges link the whole-country with road and railway communication maintaining a uniform flow of people, goods and other essential commodities. The necessity of bridges may therefore be summarized a follows:
(1) Bridges enable the free flow of traffic during monsoons and other periods of inclement weather.
(2) Bridges provide addition communication facilities.
(3) The development of the backward districts which may be rich agriculturally critically depends on the existence of bridges.
(4) Bridges provide more socio-economic benefits to the people.
(5) Bridges also enable movement of troops and military vehicles during hostilities.
2. Site investigation
Before a bridge is constructed, a suitable site is selected based on certain factors which have bearing on the economy and stability of the bridge. Reconnaissance is therefore made and the following data are recorded to determine the feasibility of bridge construction.
The factors to be considered while selecting the site for a bridge are the following:
(1) The bridge should cross the river at right angles to the direction of flow of stream or river water so as to minimize the length of the bridge.
(2) The banks on either sides of the river should have firm soil and be straight and well-defined. This will increase the stability of the bridge and reduce the possibility of the erosion of the banks. Also, the soil need not be stabilized or given any other treatment which will increase the cost.
(3) The selected site should be at a place where the river is narrow and the flow is a streamlined one without serious whirls and cross currents. Small width reduces the length of the bridge which means less cost of construction and maintenance. If the flow of water is uniform and parallel, it is a reliable guard against scour.
(4) Precautions should be taken to see that the selected site should be far away from where the river is likely to change the course. If the bridge has been constructed and the river has changed its course, the bridge will be rendered useless.
(5) Hard inerodable strata or rock should be available close to the river bed level.
(6) There should not be any sharp curves in road approaches.
3. Preliminary data to be collected
The engineer in-charge of the investigation for a bridge should collect the following information before the construction of a bridge is undertaken:
(1) V olume and Nature of Traffic Not only the present volume and nature of traffic but also
the future volume which is expected in the next ten years should be collected. The size and the type of the bridge required depend on this data.
(2) Velocity of the Stream and High Flood Level (HFL) Attained The discharge of water passing through the bridge depends on the velocity of water. It will help in designing the proper size of the waterway and pier thickness. The velocity of the stream during high flood and also during normal flow can be determined with the help of the current meter or velocity rods. The High Flood Level (HFL) will enable a bridge engineer to determine the height of free board which is the height of the road way above the HFL. It is necessary to prevent the washing away of the bridge during heavy floods.
(3) Catchment Area It is an area of that portion of watershed from which water flows and feeds the river. This is necessary to calculate the discharge of the stream.
(4) Strength and Nature of Soil and Extent and Type of Vegetation The depth of foundation for the piers and abutments depends on the strength of soil. This is determined by carrying out borings at several places and testing the soil samples. Extent and type of vegetation along with climatic conditions are also noted.
(5) Frequency of Flood Occurrence and Rainfall Details The determination of flood frequency is an important factor as most of the bridges are designed for a flood frequency of 50 years. Amount of rainfall and the HFL are also noted for future design.
(6) Scour Depth Determination Eroding of the bed of river due to heavy discharge and the velocity of water known as scour has a great bearing on the design of the depth of foundation of piers and abutments. Hence, the extent to which the bed of the river may scour below the HFL is determined according to which the design is made.
4. Technical terms
(1) Span: It is the center to center distance between two supports.
(2) Culvert: It is a small bridge having maximum span of 6 m.
(3) High Flood Level (HFL): It is the level of the highest flood ever recorded in a river or stream.
(4) Ordinary Flood Level (OFL): It is the flood level which generally occurs every year.
(5) Low Water Level (LWL): It is the minimum water level in the dry weather.
(6) Waterway: The area of opening which allows maximum flood discharge to pass under the bridge without increasing the velocity to a dangerous limit is called waterway.
(7) Afflux: Due to construction of the bridge, there is a contraction in waterway. This results in rise of water level above its normal level while passing under the bridge. This rise is known as afflux.
(8) Free board: The difference between the H .F.L. and the level of the crown of the road at its lowest point is called free board.
(9) Head room: It is the vertical distance between the highest point of a vehicle or vessel and the lowest point of any protruding member of a bridge.
(10) Length of the bridge: The length of a bridge structure will be taken as the overall length measured along the center line of the bridge from the end to the bridge deck.
(11) Viaduct: It is a continuous structure which carries a road or railway like a bridge, over a dry valley composed of series of spans over trestled bents instead of solid piers.
(12) Causeway: It is apucca submersible bridge which allows floods to pass over it. It is provided on less important routes in order to reduce the construction cost of cross-drainage
structures. It may have vents for low water flow.
5. Classification of bridges
5.1 According to the Expected Utility of Service
The bridges are classified as temporary and permanent bridges.
(1) Temporary Bridges: The bridges which are constructed and maintained at low cost and have short span of life are called temporary low cost bridges, e.g. Timber bridges.
(2) Permanent Bridges: These are bridges which are constructed and maintained at high cost and have a long span of life. These bridges are built to last for centuries, e.g. steel bridges and R.C.C. bridges.
5.2 According to the Position of the Floor of the Bridge
Relative to formation level and highest flood discharge the bridges are classified as deck bridges, through bridges and semi-through bridges.
1) Deck Bridges When the platform of a bridge, carrying the communication route is supported at the top of the superstructure, i.e. when the superstructure of a bridge is accommodated between the high flood level and the formation level then the bridge is known as the Deck bridge.
2) Through Bridge When the platform of a bridge, carrying the communication route is suspended at the bottom of the superstructure, i.e. when the superstructure of a bridge projects completely above the formation level then the bridge is known as through type bridge.
3) Semi-through Bridge When the superstructure of a bridge projects partly above and partly below the formation level, it is known as semi-through bridge or pony bridge and is thus an intermediate type between deck and through types.
5.3 According to Materials of bridges
1) Timber bridge
2) Masonry bridge
3) Steel bridge
4) R.C.C. bridge
5) Prestressed concrete bridge
5.4 Movable Bridges
Movable bridges are constructed in order to provide headway or opening for navigation ships. The design of bridge superstructure is done in such a way that it can be moved so as to allow necessary width and clearance for the passing of ships. Following are the common types of movable bridges: vertical lift bridge, bascule bridge, and swing bridge
5.5 Culverts
A culvert is a drain or water course totally enclosed and usually carried under a road or railway track, the span of which is less then 6m. The common types of culverts are box culvert, pipe culvert and arch culvert.
A box culvert consists of one or more square or rectangular openings made of R.C.C. or masonry but R.C.C. box culverts are used widely. Box culverts are used for spans less than 4 m. A R.C.C. box culvert is a cheaper alternative for a pipe culvert. The abutments, top and bottom slabs are all cast monolithically.
Pipe culverts are most economical for small drainage crossings. These culverts are generally constructed for diameters less than 1.8 m. The pipes may be of cast iron or R.C.C. If the soil is of low bearing capacity, the pipes should be bedded in a layer of concrete. If the discharge of water is
more, more than one pipe can be used.
Arch culverts are constructed on brick or stone masonry or concrete walls for short spans of 2-3 m. Depending upon the loading, span and type of construction, the thickness of an arch may be of 20-50 cm. Above the crown of an arch, an earth cushion of at least 45 cm should be provided .
阅读材料参考译文：桥梁设计理念
1．桥梁的必要性
桥梁对于一个国家是很重要的，它的作用是可从一个地区到那些被水阻断的地区运输材料，因此可使发展所需重要物资统一流动。在战争期间，物资被迅速地通过铁路桥运送到前线，桥梁用公路铁路联结整个国家，使之成为人民、商品和其它必要物质的统一体，可如下总结桥梁的必要性。
（1）桥可以在雨季或其它恶劣天气下使交通如常
（2）桥提供额外交通便利
（3）落后的但农业发达的地区的发展主要依靠桥梁的存在
（4）桥为人民创造更多的社会经济效益
（5）在战时，桥可以使军队及军车快速移动
2．现场调查：
桥设计之前，根据经济和桥稳定性的要求等因素选定一处合适的地址，应进行勘测并记录以下数据，从而决定桥建筑的可行性。
选择桥址需要考虑的因素如下：
（1）应根据溪流或河水方向，桥以适当角度来跨跃河流从而使桥长最短。
（2）河两岸应有地基土质良好，顺直及良好的形状，这些都会增加桥的稳定性并减少河水冲刷的可能性，若土质需要补强或其它处理，会使工程造价上升。
（3）桥址应是河道窄处并且水流顺直无漩涡，小宽度能减小桥长那意味着减少建筑及维护费用，若水流是一致的并且平行的，这点易于防止冲刷。
（4）特别注意的是选址应远离流河易发生改道之处，若桥已建成，河流已改变河道，此桥应作废。
（5）坚硬不易侵蚀的地层或岩层应与河床标高相近。
（6）在道路引道处不应有急弯。
3．应搜集的初步数据：（要搜集的准备数据）
负责桥梁调查的工程师应在桥梁建设之前搜集以下数据。
（1）交通量与交通性质，不仅是现在的交通量及交通性质，同时包括未来10年可预计交通量。根据这些定桥长及桥型。
（2）水流速度及最大洪水位高，桥梁泄水能力取决于水的流速，此数据有助于设计河道尺寸及墩的厚度，常水位及洪水位下的流速可用测速仪及测速杆来测定，最大洪水位可便于桥梁工程师选定安全高度，这里的安全高度是指最大洪水位以上的行车道的高度。它对于防止桥梁在洪水期间被冲毁是必要的。
（3）汇水面积：指河流支流所过的比例区域，这时有必要计算水流的流量。
（4）土的强度、土质、植被的范围和种类，墩与桩深度取决于土的强度，土的强度按多处取土并试验土样可知，植被范围及种类都可根据气候条件可知。
（5）洪水发生频率及雨水明细，洪水频率作为一个重要因素,多数桥梁都按50年一遇洪水来设计，为将来的设计指定雨量和最大洪水高度。
（6）冲刷深度，由于大水量和水的流速造成的河床侵蚀,即冲刷深度,应在墩台设计埋深以上一定值，因此，在最大洪水位冲刷下的河床范围应根据设计来确定。
4．术语
（1）跨度：两支点中心之间的距离。
（2）涵洞：是一座最大跨度为6m的小桥。
（3）最大洪水位：据记载河流及水流的最高洪水位。
（4）一般洪水位（OFL）：每年发生的一般洪水水位。
（5）低水位（LWL）：在枯水期时的最低水位。
（6）河道：允许最大洪水在桥下排出，而没有增加危险临界速度的开放区域，称为河道。
（7）雍水：由于桥的施工导致河道中水流断面收缩，当水在桥下通过时产生水位高于通常水位的现象，这种上升被称为雍水。
（8）安全高度：在最大洪水位与最低处路面路拱标高的差值被称为安全高度。
（9）净空高度：在车辆或船只的最高点与任何突出的桥部件最低点之间的垂直距离。
（10）桥长：桥结构的长度一般作为全长，全长是沿桥中心线从桥头测至桥尾的长度。
（11）高架桥：像桥那样承担着公路或铁路的连续结构，由支架支撑代替固体墩台，一般由多跨组成跨跃无水河谷。
（12）漫水桥：是一座让水穿过的暗桥，它为次重要的路线提供方便，同时可降低桥的排水系统造价，一般有低水位水流的竖井。
5．桥的分类：
5.1根据工作的期望应用
桥可分为临时桥和永久桥。
（1）临时桥：此桥是低造价修建和维护而成，桥的使用年限短，因此被称为低造价临时桥，如木桥。
（2）永久桥：此桥为高造价修建和维护，一般有长的使用期，此桥为了可使用几个世纪而修建，如钢桥及钢筋混凝土桥.
5.2根据行车道位置划分：
与结构物标高和高洪水位排水相关，桥可划分为：
1）上承式桥：当桥支承的路线在桥梁上部结构的上面,即桥梁上部结构在最大洪水位与桥面标高之间，此种桥称为上承式桥。
2）下承式桥：桥支承的路线在上部结构的下面，如：桥上部结构工程完全处于桥面标高之上的桥被称为下承式桥。
3）中承桥：当桥梁上部结构部分在桥面标高的上面,部分在桥面标高的下面，此桥称为中承桥，处于上承式桥和下承式桥之间。
5.3按材料划分桥梁
1）木桥
2）圬工桥
3）钢桥
4）钢筋砼桥梁
5）预应力混凝土桥梁
5.4 移动式桥
移动式桥是为了给航船提供水道而修建的，桥的上部结构设计是以特殊方式设计，使之可以移动以达到必须的宽度和深度足以使船穿过，如下为开合式桥的一般形式：垂直移动桥，开合式桥，旋转型桥梁。
5.5 涵洞
涵洞是一个完全封闭的排水或过水结构，它常常埋在公路及铁路之下，它的跨径一般小于6m，一般为箱涵，管涵及拱涵。
箱涵由一个或多个方形或矩形断面组成，一般由钢筋砼或圬工制成，但钢筋砼应用较广，当跨径小于4m时，采用箱涵，对于管涵，钢筋砼箱涵是一个便宜的代替品，墩台、上下板都全部一致浇筑。
对于小型排水通道，管涵是最经济的，常做成直径小于 1.8m，管涵一般由圆钢和钢筋砼制成，若土的强度低于要求强度时，管涵应外包一层砼，如果水量大时，应采用多道涵管。
对于2-3m的小跨径排水结构，应采用砖石圬工或砼墙的拱涵，依据荷载，跨径和构造物种类，拱涵厚度可以为20-50cm之间，在拱顶以上，应保证铺设至少45cm土层。
习题答案
I. True or False
1. F
2. F
3. T
4. T
5. F
II. Reading and Comprehensions
1. B
2. A
3. C
4. B
5. B
6. D
7. D
8.C
9. A 10. A
III. Vocabulary Practice
1. C
2. C
3. D
4. B
5. C
6. A
7. D
8. C
9. A 10.C
Ⅳ. Translating into English
1. A bridge is a structure providing passage over an obstacle such as a valley, road, railway, canal, river.
2. The lowest artificially built parts of piers, abutments, etc. which are in direct contact with the subsoil supporting the structure are called foundations.
3. Arch bridges are often used because of their pleasing appearance. These are more graceful and suited for deep gorges with rocky abutments.
4. Superstructure of a suspension bridge consists of two sets of cables over the towers, carrying the bridge floor by means of suspenders.
5. Modern cable-stayed bridges present a three-dimensional system consisting of stiffening girders, transverse and longitudinal bracings.
正文译文：
桥梁
桥梁是一种工程结构物，它提供了跨越山谷、道路、铁路、沟渠、河流等障碍物的通道，不封堵下面的通道。这种通道早用在道路、铁路、沟渠、管线、自行车道或人行道上。
从事桥梁设计、施工组织及桥梁维修的土木工程的一个分支被称为桥梁工程。
1.桥梁的组成
图14-1a）为桥梁立面图，图14-1b）为桥梁平面图。桥梁主要可分为两大部分：上部结构和下部结构。桥梁的上部结构可类比为一幢建筑物的屋顶，下部结构可类比为支撑屋顶的墙、柱和基础。
上部结构是由支撑交通路线的结构构件构成。这样在支座标高以上的护栏、缘石以及由梁、拱及缆索等结构体系支撑的桥面结构构成了上部结构。
下部结构是上部结构的支撑体系。它包括墩、台、翼墙和墩台基础。
桥梁结构其他主要部分是引道、支座和河流调治工程，如散水、桥台坡面加固等。桥梁
的部分重要构件在本节作出解释。
桥墩：设置在桥梁的两端支撑（桥台）之间并建在河床中，减少跨度并承受驶过桥梁的全部荷载。桥墩座落在河床之下具有坚硬土质处的基础之上。
桥台：桥梁上部结构的端部支撑称为桥台。可使用砖砌体、砌石结构、钢筋混凝土或混凝土预制块。它兼有桥墩和挡土墙的作用。桥台的高度与桥墩相同。桥台的功能如下：（1）将上部结构传来的荷载传给基础
（2）给出桥梁上部结构的最终路基标高
（3）挡住引道的路堤土方
翼墙: 位于桥台两端挡住桥梁引道填土的墙体称为翼墙。通常，翼墙的横断面逐渐减小。翼墙的设计是独立的。一般将翼墙的迎水面做成竖直的。
基础：直接与支撑结构的地基接触的桥墩、桥台等人工建筑的最低部分称为基础。
影响基础选择的因素有土的类型、土的性质、桥梁的类型、水流速度及施加在桥梁上的荷载。
井筒基础是印度最长使用的桥梁基础。这种基础由一个单独的大直径井筒或一组较小的圆管或其它形状的井筒构成。
引道：这指桥梁两端联接线的长度。引道是路堤还是路堑取决于桥梁的设计。按照印度道路委员会的建议，引道在桥梁的每一侧必须为最小16米长的直线。引道的作用是将联接线抬高至桥梁的桥面标高。
护栏和缘石：护栏设置在桥梁的两侧，防止车辆掉入河流中。还应设置人行道以便行人行走而不妨碍密集的车辆交通。
为防止车辆撞击栏杆或护栏，在路面的两侧沿人行道的边缘设置涂成白色的缘石。在填方地段，为防止车辆从路堤的侧面跃出，也应沿引道的两侧设置缘石。
梁的支座: 纵梁一定支撑在桥墩上，而桥墩将承受来自纵梁上荷载的推力，为此主梁端部应支撑在合适的支座上，同样也可设置水泥混凝土垫块，使荷载可以均匀的分配给所支撑的结构上。由于纵梁受热和受冷时会产生伸长和收缩，在桥台的端部应设置转轴使主梁可以转动而不引起主梁弯曲的发生。
2. 桥梁类型
2.1 拱桥
拱桥由于外观优美而常被采用。在深谷的岩石台座上修建拱桥更美观并且更适合。拱桥可以很经济的做到250米一跨。这种桥型中，行车道修建在支撑在桥墩或桥台的拱圈上。拱桥的一个事例是跨越Niagara河单孔跨径为290米的彩虹桥。
拱桥的优点是：拱圈的任何部位不受弯，冲击力引起的振动最小，外形优美。
2.2 板桥
这是最简单的以及最容易施工的钢筋混凝土桥梁。板桥的经济跨径通常可达9米。板的厚度非常一致，因此模板工艺简单。尽管所需的混凝土和钢材的数量较大，但施工非常简单，且容易浇筑。
2.3 T梁和板桥
这包括支撑在桥墩和桥台上的T梁。桥面板支撑在T梁上。这种桥梁适于跨径9-20米。T梁造价便宜，所需材料数量较少。例如印度最长的钢筋混凝土T梁桥是位于Goa 的Advai 桥，该桥跨径达35米。
2.4 弓弦式梁桥
当桥下净空高度较大时，使用弓弦式梁桥较为经济。主要由一道类似于弓形的拱圈和一道类似于弓弦的系梁构成。荷载的主要部分被梁承担，拱作用在桥台上的推力将受到限制。因此，桥台无需太重。行车道实际上被竖直吊杆悬吊于拱肋之上，如图14-2所示。这类桥
型的跨径为30-45米
2.5 悬索桥
悬索桥的上部结构为通过桥塔的两道主缆构成，通过吊杆悬吊着桥面结构，如图14-3所示。这类桥梁最适于交通量较小的跨径大于600米的大跨桥梁。这类桥梁是柔性的，因此，竖直振动比其他桥型大。全部荷载将被锚固于地基的主缆承受。
2.6 斜拉桥
斜拉桥按照这样一种结构体系建造，正交板和连续梁被斜拉索支撑，即通过或固定于主墩上的桥塔上的斜钢索。现代斜拉桥是由加劲梁、横向和纵向系杆组成的三维体系。正交板以及支撑部分，像受压桥塔和受拉的斜索组成的三维体系。这样的三维结构的重要特征是横向结构完全参与了主要的纵向结构的受力。这意味着结构的惯性矩显著增大，主梁的高度可以减小，节省钢材。
2.7 钢桥
钢桥通常用于支撑公路、水、油及气体管线、铁路等，可分类如下：
2.7.1 钢桁架桥
钢桁架桥常用于长铁路桥，并且几乎不受风载影响。由于组成构件较轻，因此架设钢桁架桥较为容易。构件中主要为轴向力。钢桁架桥通常用于以下情况。
2.7.2 钢刚架桥
这类桥梁在门架的顶部设置行车道，这类桥梁无需支座和固定设施。桥下净空较大，无需重力式桥台。
2.7.3 板梁桥
板梁桥通常用于荷载较大，跨径较大的情况。因此主要用于铁路桥。跨径可达20米。为了增加侧向稳定性，使用四块角连接的板组成的箱梁。
2.7.4 钢拱桥
在无法施工中间桥墩时，可使用钢拱桥。这类桥可用于非常长的跨径，即可达150米。钢拱桥既可以做成空腹式也可做成桁架式，如图14-4所示。
2.7.5 钢弓弦式梁桥
对于钢弓弦式梁桥，为承受推力，在拱圈的两端设置钢拉杆。在这类桥梁里，拱肋上吊杆悬吊着行车道。